1. Field of the Invention
This invention relates to the gasification of black liquor. In one of its more particular aspects this invention relates to a process in which aqueous black liquor is gasified to produce a combustible gas.
2. Prior Art
In the production of pulp and paper using the sodium-based sulfate and sulfite processes digestion of wood with aqueous alkaline solutions results in the production of a byproduct which is known as spent or black liquor, hereinafter referred to as black liquor. This byproduct is considered a waste material and must be converted into useful products in order to realize economies in the overall pulping process. In particular, it is dsired to regenerate sodium sulfide, which can be used to reconstitute active solutions for the pulp digestion step in the process. In addition, it is desirable to utilize black liquor as an energy source.
The most widely practiced method of processing black liquor makes use of the Tomlinson recovery boiler. In this process concentrated black liquor is burned in the furnace of a specially designed boiler to produce steam; a molten salt product called "smelt," which contains sodium carbonate and sodium sulfide; and noncombustible flue gas, which, after suitable cleaning, is vented to the atmosphere. The process has served the pulp and paper industry for about fifty years, yet it has serious deficiencies. The large volume of flue gas is difficult to clean and can constitute an environmental problem; all recovered energy is in the form of steam, which has limited utility; explosions can occur if the boiler tubes leak and cause water to contact the smelt; and the reduction of sulfur compounds to sulfide is incomplete.
Various processes involving alternatives or improvements to the Tomlinson boiler have been used or proposed for converting black liquor to useful products.
U.S. Pat. No. 1,808,773 discloses a process which utilizes a black liquor recovery furnace having two zones of combustion. In the first high temperature combustion zone black liquor sprayed into the furance is dehydrated and substantially completely burned. In the second zone, located between the first zone and the bottom of the furnace, an additional quantity of black liquor is sprayed into the furnace along with sodium sulfate. In this second zone water is removed from the black liquor by evaporation, and partial combustion of the black liquor results in the formation in the bottom of the furnace of a smelting bed of spongy carbon, mixed with alkali residues from black liquor and added sodium sulfate. Reducing conditions maintained in the bottom of the furnace result in the reduction of sulfate to sulfide. Although this process provides an alternative to use of the Tomlinson recovery boiler the necessity for two discrete combustion zones requires a cumbersome apparatus and the absence of any provision for heat recovery results in the loss of the heating value of the black liquor.
U.S. Pat. No. 2,056,266 describes the use of a combined smelter and boiler much like the Tomlinson boiler for recovering alkali metal values from black liquor and utilizing the heat content thereof. Dried black liquor solids are fed to a fuel bed zone where they are burned in a reducing atmosphere with the result that partially burned gases rise from the fuel bed. These partially burned gases then are completely burned by introducing a stream of air into a combustion zone above the bed. The combustion zone contains boiler tubes for the production of steam. Flue gases produced in the combustion zone are allowed to rise and an inert gas is blown down on the fuel bed to prevent entrainment of solids in the gases rising from the fuel bed and to create a distinct line of separation between zones. Fused alkaline values are drained from the bottom of the bed. Although this process provides means for recovering alkali metal values from black liquor and utilizing at least some of the heat content thereof, the process requires conversion of black liquor to black liquor solids prior to introduction into the fuel bed zone. In addition, the process has many of the disadvantages inherent in the use of the Tomlinson boiler.
U.S. Pat. No. 2,182,428 discloses a process for drying waste liquors by spraying the liquor to be evaporated upon the surface of a heat transfer medium such as oil, tar, pitch, asphalt or wax. Since the heat transfer medium is inert and no combustion or reduction reactions occur, the waste liquors are merely evaporated without recovering any useful product from the evaporated liquors.
U.S. Pat. No. 4,441,959 discloses a process for recovering heat and chemical values from spent pulping liquors which utilizes a fluidized bed reaction chamber. A concentrated spent pulping liquor is combusted with air in a fluidized bed comprising a plurality of inert solid particulate materials, at least one of which is a finer particle size than another. Following combustion, the particulate materials of finer particle size are treated in an external fluidized bed heat exchanger to recover heat and to separate the finer particles from gaseous and solid products produced in the combustion. The solid products are thereafter subjected to treatment in a molten salt reducer, which results in the production of a smelt containing sodium sulfide and other salts. The gaseous products essentially comprise a noncombustible flue gas, the heat content of which is used to produce steam. The resulting cooled flue gas, following suitable purification, can be released to the atmosphere. Although this process recovers some of the heat and chemical values from spent pulping liquors, since the solid combustion products are not reduced in the fluidized beds, a separate molten salt reducer is required, adding to the complexity of the process.
Processes are also available for producing a combustible gaseous product from the gasification of various carbonaceous feed materials.
U.S. Pat. No. 3,916,617, assigned to the same assignee as the present invention, describes the use of a molten salt to produce a low Btu gas from the gasification and partial oxidation of a carbonaceous material.
U.S. patent application Ser. No. 350,560, assigned to the same assignee as the present invention, described the gasification of dried black liquor solids in a molten salt pool. In this process, a combustible offgas is produced and a high level of reduction of the sulfur content of the black liquor solids to sulfide is realized. However, it is necessary to dry the black liquor to form the black liquor solids required as feed to the molten salt pool which increases the complexity and cost of the process.
U.S. patent application Ser. No. 486,274, assigned to the same assignee as the present invention, describes the gasification of aqueous black liquor using a molten salt pool. In this process an oxygen-containing gas is introduced beneath the surface of a molten salt pool comprising an alkalai metal carbnate and an alkali metal sulfide contained within an enclosed gasifier vessel at a rate sufficient to produce a high degree of turbulence in the molten salt pool. Black liquor in the form of a coarse spray is introduced into the rising hot gases above the pool, whereby water is evaporated from the aqueous black liquor into the hot gases to produce a reduced temperature product gas and dried black liquor solids, which fall onto the surface of the pool and are dispersed therein. The dried black liquor solids are converted in the pool into a hot combustible gas, which rises out of the pool, and alkali metal salts, which merge with the existing salts in the pool. A stream of product gas with a dry basis heating value of at least about 90 Btu/scf is withdrawn from the gasifier vessel together with a molten salt product in which the sulfur content is at least about 90% in the form of alkali metal sulfide. Although the process of this invention produces the desired results of providing a combustible gas and a molten salt product in which alkali metal sulfide predominates, the process is subject to the problems of corrosion and destruction of containment materials inherent in the use of turbulent pools of molten salts. Another problem encountered in the use of a turbulent pool of molten salt is entrainment of molten salts in the gases rising out of the pool, which problem can be minimized only at the expense of limiting the gas velocity through the pool.
It is accordingly an object of the present invention to provide a process for the gasification of aqueous black liquor which has none of the disadvantages of the prior art.
Another object of this invention is to provide a process which is capable of conveniently recovering a major portion of the energy and chemical content of black liquor.
A more particular object of this invention is to provide such a process in which a combustible gaseous product is produced and in which the sulfur content of the resulting salt product is predominantly present in the form of sulfide.
Another object of this invention is to provide a process which does not require the use of a turbulent pool of molten salt.
Another object of this invention is to provide such a process in which the combustible gas has a higher heating value of at least about 90 Btu/scf.
Other objects and advantages of this invention will be apparent from the following detailed description.